The demand for a large volume of fuel for transportation sector, for industrial use and increasing modern societal life-style is mainly supplied by petroleum based derivatives. Both industrial processing of fossil resources and its commercial utilization damaged the environment. Since the steady increase in fossil fuel requirements and decrease in natural fossil resources are at alarming rate that have urged to go for alternative and renewable energy resources. To full-fill the future energy requirements (at the same time) development of eco-friendly process for hydrogen production occupies first place as it has proven as cleaner resource and significant improvement in energy efficiencies up to 60% compared to petroleum derived fuels (30-35%) in internal combustion engine.
Several methods are existing for hydrogen production like steam reforming of natural gas, partial oxidation of hydrocarbons, auto thermal reforming of glycerol, gasification of coal and biomass, photo biological production and photocatalytic water splitting [Acta Geodyn. Geomater, “The resources and methods of hydrogen production”. Vol. 7, No. 2,158 (2010) pp 175-188, Chi-Hung Liao, Chao-Wei Huang and Jeffrey C. S. Wu, “Hydrogen Production from Semiconductor-based Photocatalysis via Water Splitting”. Catalysts Vol. 2 (2012) pp. 490-516].
Some of the above methods require higher temperature and pressure the resources are non-renewable in nature and potential to damage the environment. Photocatalytic water splitting process demonstrated as promising one for hydrogen production as it works well under ambient conditions utilizing renewable resources like water, sunlight in the presence of semiconductor photocatalysts [M. Anpo and P. V. Kamat (Eds), “Environmentally Benign Photocatalysts: Applications of Titanium-oxide based Materials” Springer, New York (2010)]. Among the important criteria identified for efficient photocatalytic water splitting are: (i) energy band configuration and (ii) surface properties of semiconductor photocatalyst.
The energy bands were systematically tuned to their desired energy level by modulation of valence band and conduction band. The surface properties were controlled by facilitating charge-carriers transfer to the surface by using hybrid catalysts, composite catalysts, using noble metal loaded catalysts and nanostructured catalysts. They greatly influence the optical and surface properties of the photocatalyst.
Nano structured photocatalysts exhibited improved performance than nanoparticles in water splitting process [A. Kudo and Y. Miseki, “Heterogeneous Photocatalyst Materials for Water Splitting” Chem. Soc. Rev., Vol. 38 (2009) pp. 253-278; X. Chen, S. Shen, L. Guo and S. S. Mao, “Semiconductor-based Photocatalytic Hydrogen Generation” Chem. Rev., Vol. 110, (2010) pp. 6503-6570; H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri and J. Ye, “Nano-photocatalytic Materials: Possibilities and Challenges” Adv. Mater., Vol. 24 (2012) pp. 229-251; Y. Quab and X. F. Duan, “Progress, Challenge and Perspective of heterogeneous photocatalysts”, Chem. Soc. Rev., Vol. 42 (2013) pp. 2568-2580; C. Huang, W. Yao, A. T. Raissi and N. Muradov, Development of efficient photoreactors for solar hydrogen production, Solar Energy, Vol. 85 (2011) pp. 19-27]. Particularly, 1-D TiO2 nanostructure with tubular and hollow space inside (porous) is of great potential for photocatalysis applications [D. V. Bavykin, V. N. Parmon, A. A. Lapkin and F. C. Walsh, “The effect of hydrothermal conditions on the mesoporous structure of TiO2 nanotubes”, J. Mater. Chem., Vol. 14 (2004) pp. 3370-337]. Such catalysts showed a large surface area, extended energy band potential and electron delocalization along the uni-directional axis. Enhancement of H2 production rate with TiO2 nanostructures modified with dopents, sensitizers, co-catalysts and scavengers were reported [M. V. Shankar and J. Ye, “Inorganic alkaline-sols as precursors for rapid synthesis of ETS-10 microporous titanosilicates and their photocatalytic reforming of methanol under visible-light irradiation” Cat. Comm., Vol. 11 (2009) pp. 261-265; J. Krishna Reddy, G. Suresh, C. H. Hymavathi, V. Durga Kumari and M. Subrahmanyam, “Ce(III) species supported zeolites as novel photocatalysts for hydrogen production from water”, Cat. Today. Vol. 141 (2009) pp. 89-93; Z. Jin, X. Zhang, Y. Li, S. Li and G. Lu, “5.1% Apparent quantum efficiency for stable hydrogen generation over eosin-sensitized CuO/TiO2 photocatalyst under visible light irradiation” Catal. Commun., Vol. 8 (2007) pp. 1267-1273; L. Zhang, B. Tian, F. Chen and J. Zhang, “Nickel sulfide as co-catalyst on nanostructured TiO2 for photocatalytic hydrogen evolution”, Int. J. Hydrogen Energy, Vol. 37 (2012) pp. 17060-17067; F. Guzman, S. S. C. Chuang and C. Yang, “Role of Methanol Sacrificing Reagent in the Photocatalytic Evolution of Hydrogen”, Ind. Eng. Chem. Res. Vol 52 (2013) pp. 61-65]. Copper based TiO2 photocatalysts showed efficient H2 generation superior to several noble-metal loaded TiO2 systems [X. Qiu, M. Miyauchi, H. Yu, H. Irie and K. Hashimoto, “Visible-Light-Driven Cu(II)-(Sr1-yNay)(Ti1-xMox)O3 Photocatalysts Based on Conduction Band Control and Surface Ion Modification”, J. Am. Chem. Soc., Vol. 132 (2010) pp. 15259-15267; H. Yu, H. Irie and K. Hashimoto, “Conduction Band Energy Level Control of Titanium Dioxide: Toward an Efficient Visible-Light-Sensitive Photocatalyst” J. Am. Chem. Soc., Vol. 132 (2010) pp. 6898-6899; S. Xu and D. D. Sun, “Significant improvement of photocatalytic hydrogen generation rate over TiO2 with deposited CuO”, Int. J. Hydrogen Energy Vol. 34 (2009) pp. 6096-6104; L. S. Yoong, F. K. Chong and B. K. Dutta, “Development of copper-doped TiO2 photocatalyst for hydrogen production under visible light”, Energy. Vol. 34 (2009) pp. 1652-1661; S. Xu, J. Ng, X. Zhang, X. Zhang, H. Bai and D. D. Sun, “Fabrication and comparison of highly efficient Cu incorporated TiO2 photocatalyst for hydrogen generation from water”, Int. J. Hydrogen Energy., Vol. 35 (2010) pp. 5254-5261; W. J. Foo, C. Zhang and G. W. Ho, “Non-noble metal Cu-loaded TiO2 for enhanced photocatalytic H2 production” Nanoscale, Vol. 5 (2013) pp. 759-764; W. Fan, Q. Zhang and Y. Wang, “Semiconductor-based nanocomposites for photocatalytic H2 production and CO2 conversion”, Phys. Chem. Chem. Phys., Vol. 15 (2013) pp. 2632-2649. a) S. Xu, J. Ng, A. J. Du, J. Liu and D. D. Sun, “Highly efficient TiO2 nanotube photocatalyst for simultaneous hydrogen production and copper removal from water”, Int. J. Hydrogen Energy, Vol. 36 (2011) pp. 6538-6545; H. Dang, X. Dong, Y. Dong, Y. Zhang and S. Hampshire, “TiO2 nanotubes coupled with nano-Cu(OH)2 for highly efficient photocatalytic hydrogen production”, Int. J. Hydrogen Energy, Vol. 38 (2013) pp. 2126-2135; S. S. Lee, H. Bai, Z. Liu and D. D. Sun, “Novel-structured electrospun TiO2/CuO composite nanofibers for high efficient photocatalytic cogeneration of clean water and energy from dye wastewater”, Wat. Res. Vol. 47 (2013) pp. 4059-4073]. They possess significant advantages in photo conversion efficiency and promote electron-hole separation via interfacial charge transfer process. Copper oxide has several benefits such as narrow band-gap, stability, affordability and abundantly available in nature. Notably few publications on copper oxide based nano-TiO2 exhibited high photocatalytic activity under solar light and UV light irradiation [K. Lalitha, G. Sadanandam, V. Durga Kumari, M. Subrahmanyam, B. Sreedhar and N.Y. Hebalkar, “Highly Stabilized and Finely Dispersed Cu2O/TiO2: A Promising Visible Sensitive Photocatalyst for Continuous Production of Hydrogen from Glycerol:Water Mixtures”, J. Phys. Chem. C Vol. 114 (2010) pp. 22181-22189; S. Xu, A. J. Du, J. Liu, J. Ng and D. D. Sun, “Highly efficient CuO incorporated TiO2 nanotube photocatalyst for hydrogen production from water”, Int. J. Hydrogen Energy Vol. 36 (2011) pp. 6560-6568].
Disadvantage of the prior art reported here, the excitation source of catalyst is non-renewable that is Ultra-Violet (UV) light emitted by Hg lamp, which is non-renewable, total process is expensive due to various factors viz., lamps cost, its recycling, electricity, and to maintain reaction temperature, besides its deleterious effects such as carcinogenic and environmental concerns. [H. Kato, K. Asakura, and A. Kudo, “Highly Efficient Water Splitting into H2 and O2 over Lanthanum-Doped NaTaO3 Photocatalysts with High Crystallinity and Surface Nanostructure”, J. Am. Chem. Soc., Vol. 125 (2003) pp. 3082-3089; A. Naldoni, M. D. Arienzo, M. Altomare, M. Marelli, R. Scotti, F. Morazzoni, E. Selli and V. D. Santo, “Pt and Au/TiO2 photocatalysts for methanol reforming: Role of metal nanoparticles in tuning charge trapping properties and photoefficiency” Appl. Catal. B, Vol. 130-131 (2013) pp. 239-248; K. Lalitha, J. Krishna Reddy, M. V. P. Sharma, V. Durga Kumari and M. Subrahmanyam, “Continuous hydrogen production activity over finely dispersed Ag2O/TiO2 catalysts from methanol:water mixtures under solar irradiation: A structure-activity correlation”, Int. J. Hydrogen Energy Vol. 35 (2010) pp. 3991-4001; G. Sadanandam, K. Lalitha, V. Durga Kumari, M. V. Shankar and M. Subrahmanyam, “Cobalt doped TiO2: A stable and efficient photocatalyst for continuous hydrogen production from glycerol: Water mixtures under solar light irradiation”, Int. J. Hydrogen Energy Vol. 38 (2013) pp. 9655-9664., D. Praveen Kumar, M. V. Shankar, M. Mamatha Kumari, G. Sadanandam, B. Srinivas, V. Durga Kumari, “Nano-size effects on CuO/TiO2 catalysts for highly efficient H2 production under solar light irradiation” Chem. Commun., Vol. 49 (2013) pp. 9443-9446].
The catalyst synthesis involves harsh conditions, wherein TiO2 (Degussa p-25) as precursor in 10M NaOH solution and heated at 150° C. for 48 h, with post-synthesis modification and 400° C. calcination temperature. The above TiO2 precursor reported particle size 25 nm and is widely reported catalyst for various photocatalytic applications. TiO2 (DegussP-25) cost INR 1500-1800 per kg, whereas TiO2 μm-size (Merck, India) cost only INR 700-800 per kg. Hence use of TiO2 (DegussP-25) is expensive one. For CuO/TiNT, they have reported 10 wt % of Cu/Ti ratio, that is very high compared to our report, 10% Methanol is used to improve photocatalytic performance. Moreover, methanol can be directly used as fuel in Direct Methanol Fuel Cell, which is more beneficial. Methanol is a well-known carcinogenic chemical and its usage is often restricted due to clean environmental concerns. The earlier report does not claim stability of the photocatalyst. In our report, we have used ambient conditions and natural solar (renewable energy) light as light source. We have synthesized TiO2 micron-size particles (Merck) in 10 M NaOH solution and hydrothermally heated at 130° C. for 20 h & it was post-synthesis calcined at 350° C. and on the whole overall less time and lower temperature compared to previous report, that saves considerable amount of electrical energy, environment and cost as well. We have used industrial by-product glycerol as hole scavenger for enhanced H2 production. Need of use of UV lamps which are cost effective for the process of hydrogen generation and their preparation involving adsorption and calcination method is also cost effective and reproducibility of catalyst and the recycle activity and leaching of copper are not mentioned. Whereas, the advantage of the present investigation is that metal loading by simple incipient wet impregnation and utilization of renewable source of energy i.e. natural Solar light. More over higher amount of hydrogen is produced than that of earlier report. No leaching of copper and 2 times recycle activity is observed.